The present invention relates generally to a hot wire flow meter for measuring fluid amount and velocity in use with an electrically conductive hot wire which has a variable electric resistance depending on the temperature thereof. More particularly, the invention relates to a hot wire flow meter adapted for measuring fluid flow rates generally in the form of fluid pulses.
In internal combustion engines, reciprocating pump, diaphragm pump and so forth are utilized, and it is necessary to measure or determine flow rate of fluid flowing through fluid flow passages. For measuring the fluid flow rate in the internal combustion engine itself and through the reciprocating pumps, diaphragms and so forth, various fluid flow meters are utilized. Conventionally, mechanical fluid flow meters such as that using a potentiometer have contained various drawbacks and disadvantages with respect to durability, resistance against vibration, high cost and so on. Furthermore, in cases of the fluid flow meters using a potentiometer, the response characteristics of such fluid flow meters with respect to variation of the flow amount and flow velocity is relatively low due to hysterisis within the potentiometer.
A hot wire flow meter is therefore considered as being useful for measurement of the fluid amount. The hot wire flow meter output has a non-linear response charactertistic with respect to variation of the flow amount or velocity of the fluid but can follow such variation with good response time.
Therefore, the hot wire flow meter has been widely used for measuring fluid amount or velocity flowing through the fluid passage. In the prior art, the measured flow amount of the fluid in the conventional hot wire flow meter is apt to be smaller than the actual flow amount. Particularly, for measuring the flow amount of a pulsating fluid, the difference of the measured value and the actual value becomes substantial in comparison with the measurement of the steady flow, as shown in FIG. 1. In FIG. 1, the broken line Y represents the measured flow amount of the pulsating fluid indicated by the output voltage of the conventional hot wire flow meter. As apparent from FIG. 1, the output Y of the hot wire flow meter is lower than that corresponding to the actual flow amount as represented by the solid line X. In case that the hot wire flow meter is applied for measuring an intake air flow rate in the air intake passage of the internal PG,4 combustion engine for indicating an engine control parameter in the electronic engine control system, such difference between the output value of the hot wire flow meter and the value corresponding to the actual flow amount may adversely influence the control operation of the engine. FIG. 2 shows a graph of the intake air flow rate as measured by the hot wire flow meter. In FIG. 2, the line X represents the output value of the hot wire flow meter with respect to the measured intake vacuum, assuming the flow in the air induction passage is a steady flow. The line X is compared with the line Y which is representative of the hot wire flow meter output in measurement of the pulsating fluid flow. As apparent from FIG. 2, in a vacuum pressure range of approximately -50 mmHg, the difference between the output values in measurement of steady fluid flow and in measurement of pulsating fluid flow becomes 20%. This difference may affect engine control operation of the electronic engine control system.